The invention relates to new chemical compounds and to use of a class of compounds comprising such new compounds as tanning agents for hydrophilic colloids such as gelatin and the like and particularly as tanning agents for hardening hydrophilic colloid layers. As used in various embodiments, the invention relates to thermography and photography, particularly to hardening of gelatin and the like layers in thermographic and photographic recording elements by means of the new class of tanning agents.
Tanning agents of the class used in the present invention are distinguishable over usual tanning agents known in the art, such as formaldehyde, acrolein, glyoxal, dihydroxy ketone, fructose, mucochloric acid, and the like, in that the instant compounds do not tan gelatin compositions having a pH lower than or equal to 5 at ordinary room temperatures as the customary tanning agents do, but they become very rapid tanning agents at activation temperatures above 70.degree.C. By virtue of this selective hot-tanning property, these tanning agents will find uses in which the ordinary room temperature colloid hardeners would not be used. The new tanning agents also may be used to replace conventional hardeners in some uses. The compound 2-phenoxy-3-chloromalealdehydic acid in which, according to the general formula described below, R and R' are both hydrogen atoms, was described by Sawyer in Proc. Am. Acad. Arts and Sciences, 1893,29. 242. We have not found reference to other compounds of the class described.